Volume 41, Issue 12 - December 2006

The Aluminum Citings

A Class All its Own
Differences in the Types of Anodizing
by John B. McClatchey Jr.

When it comes to anodizing there are many options to consider. In terms of brake metal on a job, considerations include coil-anodized metal, coil-painted metal, batch-anodized metal, batch-painted metal and sheet size.
Anodized metal and painted metal present many options for use, which may often depend on the environment in which the material is exposed. Many customers call us asking for some education on anodizing or just rely on what they’ve always used, which is usually the least expensive finish.

What works best depends on the individual job. Being educated in the kinds of architectural anodizing will work to your advantage.
 
What is Anodizing?
Anodizing accelerates the oxidation of aluminum creating a protective barrier. This is done by passing an electric charge through the aluminum while the metal is immersed in an acid electrolyte bath. The longer the aluminum stays in the bath, the thicker the anodic coating. Once the anodic coating is formed, the coating is sealed in another immersion bath. A common misconception is that anodizing is a coating that adheres to the metal. The reality is that the anodic coating is part of the metal.

There are different classes of anodizing. A coil-coated product, for example, runs through one mass production. As the name implies, a coil of raw or mill-finished aluminum goes through the electrolyte bath. This mass run takes much less time to anodize than typical piece-by-piece “batch” processing. The resulting coating thickness is denser than batch-anodized material, but not as thick. Some say that coil-anodized material is as good as Class II “batch” anodizing. Even if that is the case, neither will last as long as Class I when exposed to harsh elements of salt and water.

Coil anodized material may cost much less and appear the same, but coil anodizing is not recommended for a harsh salty environment. It will appear nearly the same as more expensive anodic coatings initially, but over time the seal on the metal will break down much quicker than Class I in a harsh environment and the aluminum will begin to oxidize. This appears as an unsightly chalky residue. Once installed on a job, the only way to repair the oxidized material is to remove it and have it anodized again. This, however, is impractical as caulk and other foreign elements would inhibit the material from being properly anodized. 

Durability Matters
The anodic coating on coil-anodized material is between 0.2 and 0.3 mils thick. Better options for exterior use may be Class II or Class I anodizing. Generally speaking, Class II has an anodic coating of 0.4 to 0.6 mils. Class I has an anodic coating of 0.7+ mils. Thicker anodic coatings tend to offer better durability. 

Class I anodizing should be able to withstand 1,000 hours of salt spray (that should give you an idea of how durable Class I anodizing is). Unfortunately, I have heard of companies that use coil-anodized brake metal or Class II anodized sheets and pass it off as Class I anodized material. Equipment is available that can check the anodic thickness of the metal, but more often than not the difference is never checked. Get the assurance, however you may, that the metal is the quality you requested.

Anodizing can be a tricky finish with which you must deal. There are so many factors to consider; from color variation to coating thickness, anodizing is quite unique. In the glazing business anodizing needs to be understood. You must know that for exterior use in a harsh, salt-ridden environment, Class I anodizing is practically a requirement. Your work will be admired much longer and you will have the peace of mind that your product will stand the test of time. Material that begins to chalk from corrosion will have to be replaced. Coil-coated material may be the most affordable product, but take the time to educate yourself on the attributes of all anodized finishes. 

John B. McClatchey Jr. is an account manager and third generation owner of Southern Aluminum Finishing Co. and SAF Metal Fabrication in Atlanta.

USG
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